data acquisition primer
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Table of ContentsWhat Is Data Acquisition? ................................................... 3
Who Uses Data Acquisition and Why? ............................... 4
Condition Monitoring Applications .................................... 4
PC-Based Control and Automation Applications .............. 4
Research and Analysis Applications ................................. 5
Design Validation and Verification Applications ................ 5
Manufacturing and Quality Test Applications .................... 5
Relay Types and Switch Card Options for Making
Multi-Channel Measurements ............................................ 5
Relay Types ........................................................................ 5
Electromechanical Relays .................................................. 6
High Frequency Electromechanical Relays ....................... 6
Dry Reed Relays ................................................................. 7
Solid State Relays .............................................................. 7
Switch Card Configuration Options ................................... 8
Scanner Switching ............................................................. 8
Multiplex Switching ............................................................ 8
Matrix Switching ................................................................. 9
Isolated Switching .............................................................10
Common Data Acquisition Measurement Types .............11
Temperature ......................................................................11
Typical temperature measurement applications..........11
Common temperature transducer types .....................11
Temperature measurement best practices ..................12
Strain .................................................................................12
Typical strain measurement applications ....................13
Voltage ...............................................................................14
Typical voltage measurement applications..................14
DC Voltage vs. AC Voltage ...........................................14
Common voltage measurement challenges ................14
Resistance .........................................................................15
Typical resistance measurement applications .............15
Two-wire vs. four-wire resistance measurements .......15
Common resistance measurement challenges ...........15
Current ..............................................................................16
Typical current measurement applications ..................16
DC current measurement techniques ..........................16
AC current measurement technique ............................16
Common Current Measurement Challenges ...............16
Guidelines for Selecting a Data Acquisition System ......17
Software Support for Data Acquisition Equipment ........17
At the Bench and in the Lab .............................................17
A new user experience and why it is important ...........18
Access equipment with the embedded web
interface from a web browser ......................................19
Instrument Control Software ....................................... 20
In the System ....................................................................21
Leverage SCPI commands to control the system .......21
IVI and LabVIEW instrument drivers ............................21
FPO ...............................................................................21
FPO ...............................................................................21
Optimizing the system ................................................. 22
Example Application ........................................................ 23
FPO ...............................................................................24
FPO .............................................................................. 26
Conclusion ........................................................................... 28
References .......................................................................... 28
Appendix A: Keithley Data Acquisition Systems
Overview .............................................................................. 28
Appendix B: Software Support Tools Compatible
with Keithley Instruments .................................................. 28
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Data Acquisition Primer: An Introduction to Multi-Channel Measurement Systems
What Is Data Acquisition?Although defining concepts like data acquisition, test, and
measurement can be complicated, systems used to perform
these functions typically share several common elements:
• A personal computer (PC) that is used to program and control the equipment, and to save or manipulate the results after they are acquired. The PC is also often used for supporting functions, such as real-time graphing or report generation.
• A test or measurement system based on data acquisition plug-in boards for PCs, external board chassis, discrete instruments, or a combination of all these. Each of these options has its own set of pros and cons, but these can only be determined after weighing which characteristics (sampling rate, channel count, accuracy, resolution, cost, traceability, etc.) are most critical to a specific application.
• The system can perform one or more measurement and control processes using various combinations of analog
input, analog output, digital I/O, or other specialized functions. Measurement and control can be distributed between PCs, stand-alone instruments, and other external data acquisition systems.
Part of the challenge of differentiating between terms like
data acquisition, datalogging, test and measurement,
and measurement and control comes from the difficulty
of distinguishing the different hardware types in terms
of operation, features, and performance. For example,
some stand-alone instruments include card slots and
embedded microprocessors, and support building high-
channel-count, high-throughput test systems with high
measurement accuracy.
For the sake of simplicity, let us define data acquisition and
control broadly to refer to a wide range of hardware and
software solutions capable of making measurements and
controlling external processes. Figure 1 illustrates the various
elements that a data acquisition system might include.
Figure 1. Elements of a data acquisition system.
MeasurementComputation
Analysis
VVoltage
ICurrent
RResistance
Switching
Cards/Modules
Data Acquisition SystemSensors andTransducers
Temperature
Strain
Pressure
Humidity
etc.
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Data acquisition systems can be large or small, depending
on the number of devices or sensors to be tested with
one instrumentation setup. System costs will vary widely,
depending on system requirements such as measurement
accuracy, sensitivity, resolution, test speed, and the range
of signal types that will be encountered. The USB unit
shown here is an example of a low-cost option with marginal
accuracy that can monitor a few items with one test setup.
In contrast, the Keithley 3706A System Switch/Multimeter is
a more sophisticated system that can test up to 576 devices
with one test setup and measure with 7½-digit resolution and
high accuracy.
The reason for choosing one over the other (or something in
between) is completely user- and situation-dependent, but it
all eventually comes down to weighing the price/performance
tradeoffs involved:
• What is needed in the way of channel count, measurement accuracy, measurement speed, and possible additional features?
• Traceability – What degree of calibration is necessary for the equipment? Does the equipment need to adhere to certain quality standards as recommended or required by the automotive, aerospace, medical, or other industry?
• What does the equipment manufacturer provide in the way of support? Does their documentation promote ease of use and get the customer up and running quickly? Do they provide software or drivers and how practical are they?
Figure 2. Simple USB data acquisition tool.
Figure 3. Keithley Series 3700A System Switch/Multimeter.
Typically, an application that demands higher channel counts,
speed, traceability, calibration, etc. will demand a more
expensive data acquisition system. Although higher cost
might initially seem like a bad thing, selecting the wrong tool
for the job and regretting it later is far worse.
Who Uses Data Acquisition and Why?Data acquisition hardware and software are widely used
by engineers and research scientists in a broad range of
industries and applications to gather the information they
need to do their jobs better. The following list includes just a
few of the numerous possibilities that data acquisition allows.
Condition Monitoring Applications
• Continuous monitoring of plant equipment or other assets for extended periods.
• Spotting problems early before a system failure occurs.
• Alerting maintenance personnel when an error occurs.
PC-Based Control and Automation Applications
• Controlling processes without operator intervention.
• Regulating equipment operation.
• Automating processes using open-loop and closed-loop control.
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Data Acquisition Primer: An Introduction to Multi-Channel Measurement Systems
Research and Analysis Applications
• Characterizing and recording behaviors or properties
• Studying natural phenomena
• Researching new products and designs
Design Validation and Verification Applications
• Confirming that a product or system meets its design specifications
• Establishing evidence that a product meets its users’ needs
• Testing adherence to an industry standard
• Testing over an environmental parameter range (temperature, humidity, pressure, altitude)
• Reliability testing (HALT, HASS, AST)
Manufacturing and Quality Test Applications
• Performing functional and system-level product test
• Product soak or burn-in
• Executing end-of-line pass/fail quality testing
• Inspecting for non-compliant products and subsystems
Relay Types and Switch Card Options for Making Multi-Channel MeasurementsWhen discussing data acquisition, it is often directly tied
to the idea of using relays to switch signals. Switching
helps to increase channel count economically where signal
conditioning and analog-to-digital conversion hardware can
be shared among channels sequentially.
Designing the switching for a data acquisition system
demands an understanding of the signals to be switched
and the tests to be performed. Requirements can change
frequently, so automated systems should provide the
flexibility needed to handle a variety of signals. Even simple
systems often have diverse and conflicting switching
requirements. The test definition will determine the system
configuration and switching needs. Given the versatility that
data acquisition systems must offer, designing the switching
function may be one of the most complex and challenging
parts of the overall system design. A basic understanding
of relay types and switching configurations is helpful when
choosing an appropriate switch system.
Relay Types
Understanding how relays are configured is critical to
designing a switching system. Three terms are commonly
used to describe the configuration of a relay: pole, throw, and
form. Pole refers to the number of common terminals within
a given switch. Throw refers to the number of positions in
which the switch may be placed to create a signal path or
connection. Contact form, or simply form, is a term used to
describe a relay’s contact configuration. “Form A” refers to
a single-pole, normally-open switch. “Form B” indicates a
single-throw, normally-closed switch, and “Form C” indicates
a single-pole, double-throw switch.
COM
COM
NO
NC
COM
COM
NO
NO
NO
COMNCNO
COMNCNO
1 Form A
1 Form C
2 Form A
2 Form C
c) DPST
d) DPDT
a) SPST NO
b) SPDT
Figure 4. Relay type schematics.
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Common relay types include electromechanical, dry reed,
and solid state. Some key parameters of each relay type are
outlined in Table 1.
Electromechanical Relays
Electromechanical relays have a coil of wire with a rod
passing through the middle (forming an electromagnet), an
armature mechanism, and one or more sets of contacts.
When the coil is energized, the electromagnet attracts
one end of the armature mechanism, which in turn
moves the contacts. The following figure shows a typical
electromechanical relay with the main operating elements
labeled and a scheme for actuating the armature. The coil
must generate a strong field to actuate the relay completely.
Coil
Armature
Actuation
Contacts
Pivot Point
Figure 5. Electromechanical relay (armature).
Electromechanical relays are available in configurations
ranging from 1 Form A or B to 12 or more Form C. This type
of relay can be operated by AC or DC signals and can switch
currents up to 15 amperes, as well as relatively low-level
voltage and current signals.
Electromechanical relays are available in non-latching and
latching types. Non-latching relays return to a known state
when power is removed or lost. Latching relays remain
in their last position when relay drive current is removed
or lost. Return mechanisms (for non-latching relays) and
latching mechanisms can be magnetic or mechanical, such
as a spring.
Because latching relays do not require current flow to
maintain position, they are used in applications with limited
power. In addition, the lack of coil heating, which minimizes
contact potential, makes latching relays useful in very low
voltage applications.
High Frequency Electromechanical Relays
The capacitance between poles degrades AC signal isolation
by coupling the signal from pole to pole or relay to relay.
These capacitances within relays are a common factor that
limit the frequency of switched signals. Specialized contacts
and architecture are used in electromechanical relays to
obtain good performance for RF and microwave switching
up to 40GHz. A typical configuration is shown here, where
the common terminal is between two switched terminals. All
signal connections are coaxial to ensure signal integrity. In
this case, the connectors are the female SMA type. For more
complex switching configurations, the common terminal is
surrounded by switched terminals in a radial pattern.
Relay Type Isolation Speed1 Power Life at Rated Load
Electromechanical 107 – 1010 Ω 3 – 100 ms 10 – 100 VA 107 cycles
Electromechanical (high frequency) 60 – 130 dB 20 – 100 ms 1 – 120 W 106 – 107 cycles (no load)
Contactor 106 – 109 Ω 100 – 250 ms 100 – 4k VA 105 cycles
Dry Reed 109 – 1014 Ω 1 – 10 ms 10 – 50 VA 107 cycles
Solid State 106 – 109 Ω 100 µs – 2 ms 1 – 10 VA 1010 – 1015 cycles
Table 1. Relay Comparisons.
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Data Acquisition Primer: An Introduction to Multi-Channel Measurement Systems
CoilCoil Magnet
Rocker
Reed Contacts
ReturnSpring
SMA Connectors
LogicCircuitry
Figure 6. Electromechanical relay (high frequency).
Dry Reed Relays
Dry reed relays are also operated by energizing a coil, but in
this type of relay, the coil is wound around the switch so that
the induced magnetic field closes the switch. Figure 7 shows
a simplified representation of a typical reed relay. The switch
is made from two thin, flat strips of ferromagnetic material
called reeds with contacts on the overlapping ends. Leads
are connected to the outside ends of the reeds and the entire
assembly is sealed in a hermetic glass tube. The tube holds
the leads in place (with a small gap between the contacts
for normally open switches). Normally closed switches, less
common than normally open, are made in one of two ways.
The first method is to make the switch so that the contacts
are touching each other. The second method uses a small
permanent magnet to hold normally open contacts together.
The field from the coil opposes the field of the magnet,
allowing the contacts to open.
CoilContacts
A. Side view
B Field
B. Side (cutaway) view showing lines of electromagnetic force
Force
Force
Figure 7. Reed relay.
Solid State Relays
Solid state relays are typically comprised of an opto isolator
input, which activates a solid state switching device such
as a triac, SCR, or FET, with the latter one foremost for
measurement and the former ones for control purposes.
Reliability and longevity are some key differentiators.
Although these are the fastest switching elements when
actuation times are compared, the release, or turnoff time,
is somewhat longer. AC control is a normal application for
triacs and SCRs because the turnoff time is decreased when
the device is switched off during a zero crossing. In addition,
their isolation is limited by the leakage currents of the
semiconductor devices, and they have a high insertion loss
for low-level signals.
To learn more about switch relays, refer to Section 2 of
Switching Handbook: A Guide to Signal Switching in
Automated Test Systems, which is available at www.tek.com.
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Switch Card Configuration OptionsAs a signal travels from its source to its destination, it may
encounter various forms of interference or sources of error.
Each time the signal passes through a connecting cable or
switch point, the signal may be degraded. Careful selection of
the switching hardware will maintain the signal integrity and
the system accuracy.
Scanner Switching
The scan configuration or scanner is the simplest
arrangement of relays in a switch system. As shown in the
following figure, it can be thought of as a multiple position
selector switch.
Figure 8. Scanner - 1 x n selector switch, in this example n=4.
The scanner is used to connect multiple inputs to a single
output in sequential order. Only one relay is closed at any
time. In its most basic form, relay closure proceeds from the
first channel to the last. Some scanner systems have the
capability to skip channels.
Figure 9 illustrates an example of a scan configuration. In
this diagram, the battery is connected to only one lamp
at a time, such as in an elevator’s floor indicator system.
Another example is a scanner for monitoring temperatures
at several locations using one thermometer and multiple
sensors. Typical uses of scanner switching include burn-in
testing of components, monitoring time and temperature drift
in circuits, and acquiring data on system variables such as
temperature, pressure, and flow.
(
12
3
4
Figure 9. Scanner to indicate elevator location.
Multiplex Switching
Like the scan configuration, multiplex switching can be used
to connect one instrument to multiple devices (1:N) or multiple
instruments to a single device (N:1). However, the multiplex
configuration is much more flexible than the scanner. Unlike
the scan configuration, multiplex switching permits multiple
simultaneous connections and sequential or non-sequential
switch closures.
One example of a multiple closure would be to route a single
device output to two instruments, such as a voltmeter and
a frequency counter. The figure below illustrates another
example of multiplex switching. This diagram shows
measuring the insulation resistance between any one pin
and all other pins on a multi-pin connector. To measure the
insulation resistance between pin 1 and all other pins (2 and
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Data Acquisition Primer: An Introduction to Multi-Channel Measurement Systems
3), close channels 2, 3, and 4. This will connect the ammeter
to pin 1 and the voltage source to pins 2 and 3. The insulation
resistance is the combination of R1-2 and R1-3 in parallel as
shown. Note that in this application, more than one channel is
closed simultaneously in non-sequential order.
Pin 1
Pin 2
Pin 3
R1-2 R1-3
Connector
Ch. 1
Ch. 2
Ch. 3
Ch. 4
Ch. 5
Ch. 6
Ammeter
HI
LO
VoltageSource
Figure 10. Multiplex switching used to test the insulation resistance of multi-pin connector.
Matrix Switching
The matrix switch configuration is the most versatile because
it can connect multiple inputs to multiple outputs. A matrix
is useful when connections must be made between several
signal sources and a multi-pin device, such as an integrated
circuit or resistor network. Using a matrix switch card allows
connecting any input to any output by closing the switch at
the intersection (cross point) of a given row and column. The
most common terminology to describe the matrix size is M
rows by N columns (M×N). For example, a 4×10 matrix switch
card has four rows and ten columns. Matrix switch cards
generally have two or three poles per cross point.
As shown in Figure 11, a 5VDC source can be connected to
any two terminals of the device under test (DUT). A function
generator supplies pulses between another two terminals.
Operation of the DUT can be verified by connecting an
oscilloscope between yet another two terminals. The DUT pin
connections can easily be programmed, so this system will
serve to test a variety of parts.
5VDCSource
FunctionGenerator
Oscilloscope
+
–
1 2 3 4 5 6 7 8Columns
1
2
3
4
5
6
DUT
Rows
Figure 11. 6×8 one-pole matrix example.
When choosing a matrix card for use with mixed signals,
some compromises may be required. For example, if both
high frequency and low current signals must be switched,
take extra care when reviewing the specifications of the card.
The card chosen must have wide bandwidth as well as good
isolation and low offset current. A single matrix card may
not satisfy both requirements completely, so the user must
decide which switched signal is more critical. In a system
with multiple cards, card types should not be mixed if their
outputs are connected together. For example, a general-
purpose matrix card with its output connected in parallel with
a low current matrix card will degrade the performance of the
low current card.
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Isolated Switching
The isolated or independent switch configuration consists
of individual relays, often with multiple poles, with no
connections between relays. Figure 12 illustrates a single
isolated relay or actuator. In this diagram, a single-pole
normally open relay is controlling the connection of the
voltage source to the lamp. This relay connects one input to
one output. An isolated relay can have more than one pole
and can have normally closed contacts as well as normally
open contacts.
Figure 12. Single isolated switch.
Given that the relays are isolated from each other, the
terminals of each channel on the switch card are independent
from the terminals of the other channels. As shown in Figure
13, each isolated Form A relay has two terminals. Two-pole
isolated relays would have four terminals (two inputs and two
outputs). A single Form C isolated relay would have three
terminals.
Ch. 1
Ch. 2
Ch. 3
Ch. 4
Figure 13. Isolated relays on a switch card.
Isolated relays are not connected to any other circuit, so the
addition of some external wiring makes them suitable for
building very flexible and unique combinations of input/output
configurations. Isolated relays are commonly used in power
and control applications to open and close different parts
of a circuit that are at substantially different voltage levels.
Applications for isolated relays include controlling power
supplies, turning on motors and annunciator lamps, and
actuating pneumatic or hydraulic valves.
To learn more about the various switching topologies,
refer to Section 1 of Switching Handbook: A Guide
to Signal Switching in Automated Test Systems,
which is available at www.tek.com.
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Data Acquisition Primer: An Introduction to Multi-Channel Measurement Systems
Common Data Acquisition Measurement TypesData acquisition systems can be configured to acquire
information on a wide range of physical phenomena, such
as pressure, flow, speed, acceleration, strain, frequency,
humidity, light, and hundreds more. However, the four most
commonly used measurement types are temperature,
voltage, resistance, and current.
Temperature
Measuring temperature accurately is critical to the success
of a broad array of applications, whether they are performed
in industrial environments, process industries, or laboratory
settings. Temperature measurements are often required
in medical applications, lab-based materials research,
electrical/electronic component studies, biology research,
geological studies, electrical product device characterization,
and many other areas.
Engineers from a number of disciplines (thermal, electrical,
and mechanical) are concerned with how their products and
devices perform in various environmental conditions. Having
the appropriate measurement equipment helps them capture
behaviors that cannot be witnessed by simple observation.
Typical temperature measurement applications
Thermal profiling: Measuring a device’s temperature over
its range of use is important to understanding the stresses
created at various temperature points and their effect on the
life of the device under test. In addition, identifying hot and
cold spots of the device during typical use can help designers
minimize thermal effects.
HALT/HASS: Highly Accelerated Life Testing and Highly
Accelerated Stress Screening are proven accelerated
product reliability testing methods focused on finding
defects in products so they can be fixed before they result
in expensive field failures. They subject a product to a series
of overstresses, including thermal ones, effectively forcing
product weak links to emerge by accelerating fatigue.
Stresses are applied in a controlled, incremental fashion while
the unit under test is continuously monitored.
Product development: Accurate temperature measurements are
critical during product development to allow engineers to understand
how the product responds in various thermal situations.
Common temperature transducer types
Thermocouples: Thermocouples (TCs) are formed by
the junction of two dissimilar metals. Several different
types, covering a wide temperature range, are available.
Thermocouple linearity varies, depending on thermocouple
type and temperature range. Of all the different types of
sensors available to measure temperature, the thermocouple
is by far the most common, mainly because thermocouples
are relatively inexpensive, extremely rugged, and can be
located long distances from the data acquisition system itself.
However, connecting thermocouples can sometimes pose a
challenge. In the case of higher-end data acquisition systems,
manufacturers will often provide direct termination of the
wires to a card or module populated with terminal blocks. An
advanced interface such as this may well include circuitry
to promote automated cold junction correction, helping to
eliminate complicated setups that include extra lengths of
wire, an external reference (such as an ice bath), and the need
to include an additional measurement point to monitor the
external reference. However, if the user purchased a standard
digital multimeter with only front or rear input selection,
they are faced with getting the best connection possible
being routed into terminals that were not designed for direct
thermocouple mounting. One solution is to use a thermal
bead thermocouple probe, where the terminations are made
using a standard banana plug interface. Some models of
multimeter do provide a specialized input for thermocouples
with the standard thermocouple connector, but, more often
than not, even these probes can be used with an adapter to
get to the more common banana jacks.
Figure 14. Thermocouple temperature transducer.
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RTDs: Resistance temperature detectors are made with
metal wires or films (most common being platinum) with a
resistance that changes with temperature. RTDs require
an external stimulus to function properly, usually a current
source. However, this current has a tendency to generate
heat in the resistive element, which adds error in the
temperature measurement. There are three different methods
that can be used when making RTD measurements, each
contributing to a higher degree of accuracy when applied,
respectively: two-wire, three-wire, and four-wire. RTDs are
significantly more accurate than thermocouples; however,
they are somewhat more costly and not as rugged.
Test Current (I)
I VM VM VM
Sense HI
Sense LO
RResistanceUnder Test
LeadResistances
VM = Voltage measured by meter
VR = Voltage across resistor (R)
Because sense current is negligible, VM = VR
RLEAD
RLEAD
RLEAD
RLEAD
Source HI
Source LO
Sense Current (pA)
=I
VMand measured resistance =I
VR
Figure 15. Schematic representation of an RTD measurement.
Thermistors: Thermistors, like RTDs, change resistance as
temperature changes. They offer higher accuracy like RTDs
and are easy to set up. Although thermistors are less linear
than RTDs, the measuring instrument (such as a digital
multimeter) will have built-in algorithms to help resolve this
and maintain the elevated accuracy. While they have a limited
range, thermistors have a quicker response to temperature
changes than the alternatives. A thermistor’s resistance
changes more for a given change in temperature than does
the resistance of an RTD.
Figure 16. RTD temperature transducer.
Temperature Sensor Comparison
Sensor Type Advantages Disadvantages
Thermocouple (TC)
Relatively inexpensive.
Rugged.
Wide temperature range.
Low output voltage.
Must use thermocouple wire to connect.
Requires cold junction compensation circuitry.
RTDAccurate.
Wide temperature range.
Low output resistance.
Requires excitation source.
Thermistor
High output.
Inexpensive.
Fast response.
Limited temperature range.
Low accuracy.
Poor long-term stability.
Temperature measurement best practices
• Take the time to weigh which sensor will be the most appropriate for the specific application.
• Consider the required measurement accuracy and speed.
• Determine how many points of measurement are required. Some temperature data loggers can scan and monitor many channels.
Strain
Studying the physical behavior of mechanical structures
frequently includes measuring strain, which is defined as
a physical distortion of an object in response to one or
more external stimuli applied to the object. Typical stimuli
include linear forces, pressures, torsion, and expansion
or contraction due to temperature differentials. Strain can
result in elongation or contraction; these phenomena are
noted by the use of a (+) sign for expansion or a (–) sign for
compression.
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Data Acquisition Primer: An Introduction to Multi-Channel Measurement Systems
Strain information is important in structural engineering
because it can be used to determine the stress present in an
object. Stress data can then be used to assess factors such
as the structural reliability and service life of the object. The
basic principle of strain measurement is also used in other
types of force-related measurements, such as pressure,
torque, and weight.
Strain is calculated as the change in length of an object
divided by the unit length of the object. Normally, this change
is extremely small in relation to the object’s length. Strain
gauges or sensors are designed to provide a measurable
signal (voltage, resistance, or current) to determine the
degree of distortion caused by the items in question.
Typical strain measurement applications
Load Cells, Pressure Sensors, and Flow Sensors: A variety
of physical phenomena can be measured with strain gauge-
based transducers coupled with specialized mechanical
elements. These phenomena include load, gas or liquid
pressure, and flow rate, among others.
A load cell is simply a packaged strain gauge designed to
measure force under different load conditions. A typical
load cell body is machined from metal to provide a rigid
but compressible structure. The load cell body is usually
designed with mounting holes or other fittings to allow the
unit to be mounted permanently on a supporting structure.
Typical transducers for pressure sensing are
electromechanical devices that combine a sensor with
mechanical elements such as a diaphragm, piston, or
bellows. The mechanism is designed to respond to a
pressure differential and expand against a spring, which
determines the range of the sensor. The pressure-sensing
element also actuates a strain or positional sensor that
produces an electrical output that can be read by an
instrument.
Gauge Length SolderTabs
Figure 17. A typical strain gauge.
Flow can be measured in a variety of ways, most of which
are indirect measurements that depend on the material’s
viscosity, conductance, or other properties. One very
common way to infer flow is to use two pressure sensors to
measure the material’s pressure drop when passing through
an orifice.
Acceleration, Shock, and Vibration: Acceleration, shock,
and vibration are important parameters in mechanical
applications, including automotive, aerospace, product
packaging, seismology, navigation and guidance systems,
motion detection, and machine maintenance. These
parameters can all be measured using a sensor known as
an accelerometer.
Acceleration is the rate of change of velocity that a mass
undergoes when it is subjected to a force. It is defined by the
formula: force = mass × acceleration.
Assuming that mass and force remain the same, then
acceleration is constant, meaning that an object’s velocity
will continue to change at a uniform rate. Acceleration is
expressed as distance per time squared, with meters/sec2
and feet/sec2 being common units. Earth’s gravity exerts a
force of 9.8m/sec2, or 32ft/sec2; this amount of acceleration
is often referred to as 1g (g for gravity). Sensors designed
to measure acceleration can measure values ranging from
millionths of a g (µg) to hundreds of g or more. They are
frequently designed to withstand overloads of thousands of g
without damage.
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Shock is a special case of linear acceleration in which the
acceleration (or deceleration) time approaches zero. In the
real world, the time cannot equal zero, but it can equal a
very small fraction of a second. The result of a large change
in velocity over a short time can produce an acceleration of
hundreds of g or more.
For shock studies, the measurement range of the
accelerometer must be able to handle the acceleration
expected in the specific shock situation. This value can
be calculated by dividing the change in velocity by the
time interval:
∆V a = ___ ∆t
Vibration is a continuing change in the position of a body,
typically occurring in a cyclic pattern with a constant or
near-constant period. Vibration is a common characteristic
of machines; it is of interest in manufacturing because its
measurement can provide an indication of the health of
machinery. Periodic monitoring and analysis of vibration
levels can reveal specific problems, such as worn bearings,
loose fasteners, or out-of-balance rotating components,
before they become severe enough to be noticed or to cause
complete failure. Vibration is a cyclic changing of position, so
it can be detected by using an accelerometer.
For more detailed information on strain measurements, refer
to the Data Acquisition and Control Handbook.
Voltage
Like temperature, voltage is very commonly measured with
data acquisition systems. Most electrical measurement
devices (voltmeters, digital multimeters, etc.) measure voltage
but offer different measurement ranges, sensitivity, accuracy,
and speed.
Typical voltage measurement applications
Monitoring the voltage output of a transducer. A transducer
converts a mechanical parameter into an electrical parameter,
typically a voltage. In addition to the temperature transducers
described previously, examples of transducers include
strain gauges, pressure gauges, humidity sensors, and
ultrasonic sensors.
Monitoring the voltage output of an electronic device or an
internal circuit of an electronic device. Voltage measurements
verify that a product or device meets its specifications
or performs as required. The product or device is tested
under varying conditions to ensure the product meets its
specifications, and numerous voltage measurements are
often required. Internal circuit voltages are monitored to
ensure that circuits perform properly under a wide range of
test conditions.
DC Voltage vs. AC Voltage
Direct current (DC) refers to a unidirectional flow of electric
charge. DC voltages can be produced by sources such as
batteries, power supplies, thermocouples, solar cells, or
dynamos. DC voltages may flow in conductors such as wires,
but they can also flow through semiconductors, insulators,
or even through a vacuum, as in electron or ion beams.
In alternating current (AC) circuits, rather than a constant
voltage supplied by a battery, the voltage typically oscillates
in a sine wave pattern that varies with time. In household
circuits, the AC voltage oscillates at a frequency of 60 Hz.
Frequency and period measurements are part of the AC
measurement family. Frequency is a measurement of the
number of cycles per second; period is a measurement of the
time period of the AC wave.
Common voltage measurement challenges
Input impedance of the voltmeter: The voltmeter has a limited
amount of impedance. When connected to the source of
the voltage to be measured, if the source impedance is of
sufficient magnitude, the input impedance could cause an
error in the measurement. Most voltmeters today offer a
choice of two input impedances, 10 MΩ and 10 GΩ. Typically,
the higher the input impedance used, the less the voltmeter
will affect the source voltage.
Resolution of the voltmeter: Voltmeters can offer anywhere
from 3½ digits to 8½ digits of resolution. For quick and crude
voltage measurements, choose a low-resolution voltmeter.
For more accurate and stable voltage measurements, use
higher-resolution instruments.
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Data Acquisition Primer: An Introduction to Multi-Channel Measurement Systems
Resistance
Resistance is another commonly measured parameter in
data acquisition systems; it can be measured by either of
two methods: constant voltage or constant current. The
constant voltage method sources a known voltage across
an unknown resistance and measures the resulting current,
then calculates the resistance. This method is widely used
for high resistance measurements (>1 GΩ).The constant
current method sources a known constant current through
the unknown resistance and measures the resulting voltage.
Then the resistance is calculated. This is normally used for
resistances <1 GΩ.
Typical resistance measurement applications
Measurement of resistive devices is performed to ensure
the magnitude of the resistance is within specification. Many
resistive devices have temperature coefficients, which means
the resistance varies with temperature. These characteristics
need to be verified over a temperature range.
Measurement of contact resistance. Contact resistance is the
resistance to current flow through a closed pair of contacts.
These types of measurements are made on components
such as connectors, relays, and switches. This resistance is
normally very small, ranging from micro-ohms to a few ohms.
Measurement of isolation resistance on multi-pin connectors
and multiple wire cables. This is done to ensure that there are
no unintended connections or shorts between independent
connector pins and wires.
Measuring the resistance or resistivity of a material such as a
polymer. The resistance can be used to gauge the material’s
ability to hold or dissipate charge that may accumulate, such
as on a static-sensitive mat.
Two-wire vs. four-wire resistance measurements
The two-wire resistance measurement technique uses the
same two leads for sourcing and sensing. This means that a
current is forced through the wires and the voltage is sensed
with the same wires. Note that the voltage is sensed at the
meter terminals and not at the resistive device. This technique
is convenient and simple to use; for most resistance
measurements, simply connect the two wires to the circuit of
interest and make the measurement.
The four-wire technique sources a current with two wires and
senses the voltage drop using a different set of two wires.
This allows the voltage to be detected at the resistance under
test. The four-wire technique has the advantage of eliminating
test lead resistance because it measures the resistive
device and not the test lead resistance. Test lead resistance
is generally several hundred milli-ohms in magnitude and
represents a large offset in a low resistance measurement.
Common resistance measurement challenges
Low resistance measurement with a two-wire measurement
device: To eliminate test lead resistance, most ohmmeters
and digital multimeters include a feature called ZERO, NULL
or Relative. Using this feature is a two-step procedure.
First, the two test leads are shorted together. Second,
enable the ZERO, NULL or Relative feature, which will
subtract the measured short-circuit resistance from every
subsequent reading.
When high resistance measurements are too low: When
making high resistance measurements (1 GΩ and higher), the
instrument setup itself can introduce problems. Follow these
steps to prevent these problems:
• Low insulation resistance: The test fixture’s insulation resistance could be in parallel with the device under test. To remedy this problem, use a test fixture and connecting cables with higher insulation resistance. A driven guard can also effectively increase shunt resistance.
• Input resistance of the voltmeter is too low for the measurement: Use the force voltage and measure current technique.
Offset current: Offset current may be caused by charge
stored in the material being measured. Adjust the meter’s
zero baseline to compensate for offset current or use the
alternating polarity method to cancel these offsets.
Low resistance thermoelectric errors: Measuring low
resistance is really a matter of measuring low voltage;
thermoelectric EMFs in the circuit are major sources of
error in low voltage measurements. To correct this, use
offset compensation. The first step in the technique is
to force the current in the positive polarity and take a
voltage measurement. Then, force a current of the same
magnitude to the negative polarity and take another voltage
measurement. Then add up the voltages and divide by two.
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This yields a “compensated” resistance measurement that
eliminates the thermoelectric effects.
The following table summarizes resistance measurement
techniques and the supplemental corrective functions that
can be applied to enhance each.
Table 2. Resistance measurement optimization techniques.
Measurement Technique
General resistance (10 Ω to 1 GΩ) Two-wire
Low resistance (<10 Ω) Four-wire, constant current technique, offset compensation, dry circuit
High resistance (>1 GΩ) Constant voltage technique, alternate polarity, guard to reduce leakage
Current
Current measurements are used to determine resistance,
power and, of course, current flow in a circuit. Unlike voltage
measurements, current measurements cannot be made by
simply placing the test leads across the sample. Instead, in a
standard shunt-style current measurement, the current path
is broken and a current shunt placed in series so that when
the current flows through the shunt, it develops a voltage.
With the shunt resistance value known and the voltage
measured, the current can be calculated.
Typical current measurement applications
• Monitoring the power consumption of a device under test (DUT) by measuring the current drawn from the DUT
• Measuring the current from transducers that have 4–20 mA outputs
• Determining the maximum current output of a device under various operating conditions
DC current measurement techniques
The traditional (shunt-style) current method requires
breaking the circuit and placing a shunt resistance in series
with the break. When the current flows through the shunt
resistance, a voltage is developed that is measured. Dividing
this voltage measurement by the known shunt resistance
yields the current. This is the standard method employed in
most current measuring circuits in instruments like digital
multimeters (DMMs). The smaller the anticipated current is,
the larger the shunt resistance should be. So, for measuring
microamps of current, usually a 100 kΩ or 1 MΩ shunt is
used. At higher current levels, the shunt resistance is lower;
for ampere-level measurements, usually a 1 Ω or even a
0.001 Ω shunt is used. It all depends on the level of current
anticipated and the voltage drop across the shunt.
The active shunt or feedback ammeter method also requires
breaking the circuit and placing the ammeter in series with
the break but inserting an input to an active operational
amplifier (op amp) rather than a shunt resistance. The
feedback of the op amp determines the level of current that
can be measured. This method is usually appropriate for low
current measurements, so it is employed in picoammeters
and electrometers.
AC current measurement technique
AC current measurements are DC current measurements
in which the AC signal is routed through an AC–to-DC
converter. The shunt-style method is generally used in AC
current measurements: the current flows through the shunt,
the voltage drop is measured and then routed through the
AC-to-DC converter. The bandwidth is important for AC
current measurements. With most AC current meters, the
bandwidth is up to about 5 kHz.
Common Current Measurement Challenges
Voltage burden is an error term in current measurements
resulting from having to insert the measurement instrument
into the circuit under test and having to build up a voltage
in the circuit to compute current. With shunt-style current
measurements, the voltage dropped across the shunt can
be significant, which can reduce the current to be measured.
With a feedback ammeter, the voltage burden is the offset
of the differential input of an operational amplifier, which
is usually very low (millivolts to microvolts). The lower the
voltage burden due to the measurement technique, the more
accurate the current measurement.
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Data Acquisition Primer: An Introduction to Multi-Channel Measurement Systems
Guidelines for Selecting a Data Acquisition SystemTo configure a data acquisition system appropriately, it is
vital to define application requirements early. Answering
these questions will provide valuable guidance during the
equipment selection process:
• What kinds of signals are to be measured? This could be voltage, current, resistance, etc.
• What is the magnitude of the signals to be measured? This could range from microvolts to kilovolts or picoamps to amps, or milliohms to gigaohms.
• How many signals are to be routed and measured? This answer can help determine how many measurement channels are required and allows narrowing down equipment options.
• What test timing issues are important? What acquisition rate is needed? How long will the tests last—minutes, hours, days, weeks? Some production tests can be completed in seconds, but the equipment operates 24/7. For this type of around-the-clock data acquisition, switch relay life must be taken into consideration. Although most relays can withstand millions of open/close cycles, the level of signals being routed can affect lifespan.
• What level of accuracy is required for each parameter in the test? There is typically some level of tradeoff between accuracy and acquisition rate. Generally, the higher the rate, the less accurate the measurements will be.
• How much data must be collected? Many data acquisition systems have internal buffers. Generally, saving data to these buffers offers the fastest method of obtaining data and is a good approach if the amount of data to be acquired is less than the instrument’s storage capacity. However, if the data must be displayed in real time on a screen, a different approach will be acquired. If the system includes a PC with a USB port, lots of data can be saved to a USB drive.
• How big is the equipment budget? Cost will always be a major factor in selecting a data acquisition system. The cost per channel is one factor in the cost of the system. What types of switching relays are required (electromechanical, reed, or solid state)? Because relays are mechanical in nature, they have limited lifespans. How often will that lifespan be reached? Factor the relay life into the cost equation.
See Appendix A: Keithley Data Acquisition Systems Overview
for more information.
Software Support for Data Acquisition EquipmentKnowing that the data acquisition equipment selected for
a task is capable of performing it properly is critical, but
learning how to use it can often be the greatest challenge.
Many users rely on a software interface to become familiar
with the basic functionality of their instrumentation and
gradually work their way up to more complex scenarios. The
software tools chosen need to balance intuitive operation
and high ease of use with sophisticated test development
capabilities to simplify the transition from ensuring proper
functionality at the benchtop to developing a series of
commands that optimize the instrument’s performance in a
production environment. Keithley products are compatible
with an array of software options, which are outlined in the
following sections.
At the Bench and in the Lab
The expression “bench software experience” refers to a
user’s hands-on interaction with a test instrument that is not
connected to an external computer. The firmware embedded
in the instrument is the first “software” the user encounters
because it is immediately available through the front panel
of the instrument on the benchtop. A traditional front panel
interface provides an array of buttons and soft keys (also
typically tied to actual buttons) that allow modifying the
settings and properties of the measurement behavior that
is active on the instrument. A multi-line alphanumeric liquid
crystal display (LCD) shows the actual menu options and
measurements to reflect the current state of the instrument.
Figure 18. A traditional benchtop instrument.
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The test and measurement industry has employed traditional
interfaces like these for decades, and several Keithley
data acquisition instruments that incorporate them are still
available. When this interface is used with a quick-start or
user’s guide (either printed or electronic), users can gain
familiarity with their instrument simply by following the steps
listed in the guide.
A new user experience and why it is important
The growing use of smartphones and tablet PCs have led
consumers to expect more from their electronic devices than
a nice-looking display. These devices not only enhance the
viewing of desired content but also allow the user to assume
a more interactive role in how the software applications
respond to them. This makes the experience more intimate
and builds a personal foundation between man and machine.
Much the same is true with test instrumentation. College and
university students are using touchscreen test equipment
as part of their coursework. Soon those students will be the
next wave of new engineers, building technologies that will
shape the world in previously unimaginable ways. Modern
instrument interfaces will not just be demanded, they will
be expected.
Keithley’s most recent products are equipped with
touchscreen interfaces in an effort to clean up the front
panel, speed instrument setup, simplify access to settings
and features, and allow users to begin analyzing results
faster. These instruments bring actions commonly used with
tablets or smartphones (swipe, poke, pinch and zoom) to the
benchtop, providing a more logical flow for how “everyday
measurement” should take place. The touchscreen interfaces
simplify getting test results quickly and with the high
accuracy. These touchscreen instruments offer:
• An intuitive interface that minimizes the user’s learning curve
• Easy navigation between set-up and configuration menus for fewer errors
• Real-time graphing, data visualization, and on-screen analysis
Figure 19. A modernized instrument and user interface.
Engineers often say, “I will use this primarily in a rack or
system with a remote interface, so the front panel is not a big
deal to me.” However, that point of view often changes when
a problem arises that forces the engineer to troubleshoot by
methodically manipulating the equipment configuration to find
the source of error. Having the power to debug an issue more
effectively and share this capability with support technicians
on the production floor can save considerable time.
The touchscreen user experience (UX) offers an alternative
to the remote software interface, removing the PC and the
Figure 20. Enhanced user interface features for a richer user experience.
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Data Acquisition Primer: An Introduction to Multi-Channel Measurement Systems
cabling to connect it to the instrument from the work area.
The set-up screens simplify test configuration in much the
same way as a secondary desktop software would and offer
graphing, cursors, and statistical information on the acquired
measurements to allow for immediate analysis of results.
Figure 21. Custom applications-based scripting.
Additionally, a flexible, smart software interface can simplify
bringing customized applications and setups directly to the
test equipment without the need for a firmware upgrade
or external software. This might include regular benchtop
setups or custom-built diagnostics tools. For example, some
users might want their data acquisition system to make the
measurements across multiple channels more viewable or
need some maintenance tools for monitoring the life of switch
card relays to alert them when they are approaching “end-of-
life” conditions prior to getting degraded performance.
Access equipment with the embedded web interface from a web browser
For those users who prefer working with a larger screen,
some test instruments also include an embedded web
server that can be used with a choice of web browsers,
including Microsoft® Internet Explorer or Edge, Mozilla
Firefox, Google Chrome, and Apple Safari. The web interface
(Figure 22) simplifies reviewing instrument status, controlling
the instrument, and upgrading the instrument over a LAN
connection.
The instrument web page resides in the firmware of the
instrument. Changes made through the web interface are
immediately made in the instrument. Some of the web tools
will allow typing in and issuing remote commands just as if
they were sent via custom control software. For instance, on
Figure 22. Accessing equipment with the embedded web interface from a web browser.
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some data acquisition systems, examples from manuals can
be run through an embedded web application that is available
on the instrument’s web interface.
Some instruments’ embedded web interfaces feature a virtual
front panel page that allows controlling the instrument from
a computer in much the same way as using the front panel.
The instrument can be operated by using a mouse to select
options. This can be a valuable feature for users who want to
configure and check in on their tests from another location.
Figure 23. Control your instrument from a virtual front panel.
Instrument Control Software
No matter how rich a user interface is built into a data
acquisition system, it can still be advantageous to work with
a piece of software that extends the control of the instrument
to an external computer. The mindset here is to avoid getting
bogged down in the minutiae of programming. The system
creator knows the kind of equipment needed for the test but
does not have the time to learn every little detail about it. He
or she knows the application, the stimuli that must be applied,
and the measurements to be made. What is needed is a
software package that promotes minimal effort in instrument
setup, and provides data analysis where results and images
can be immediately added to the project’s documentation or
formal report. Ideally, the software in question would have the
application structure already built into it. Fortunately, several
test instrumentation manufacturers have solutions for just this
situation.
Keithley’s KickStart is PC-based software that manages
both the instrument configuration and execution of tests. It
provides a simple interface to the instrument so that users
can gather test insights without becoming instrumentation
experts. Users can be up and running with their instrument(s)
in minutes as KickStart accelerates their learning curve by
automating data collection from multiple instrument without
programming. KickStart offers quick visualization of data and
tools to compare data from test run history. Such features
enable faster discovery of measurement trends and device
dependencies on environmental conditions, for example. This
aids the design engineer in making the necessary decisions
to continue on to the next stage of the product development
and aids the test engineer in validating the performance
of the device or certifying that the device complies with a
particular regulatory standard.
KickStart can capture millions of readings from the
instrument and stream them directly to PC storage media
for safe data archival. Users may save test configurations in
order to replicate previous tests quickly and even create a
library of tests for a particular device or compliance standard.
KickStart provides a variety of user conveniences for
interacting with the data so that users can make faster
decisions on what the data shows:
• Customize view of the data table to show and hide only the data of interest
• View plots of multiple measurements at once
• Assign meaningful labels to channels of the data acquisition switching card
• Get statistical overviews of data for each channel in the table
• Highlight/mark data points of interest on the graph
• Export data at any time during the test to share test updates with colleagues and stakeholders
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Data Acquisition Primer: An Introduction to Multi-Channel Measurement Systems
In the System
When the experimental setups and equipment understanding
are complete, it may be time to consolidate all test equipment.
Individual elements are mounted into a rack, customized
fixtures are created that route test paths to the DUT, and a
common piece of test software that can sequence the tests
to be executed in a production environment is generated.
The bench and lab setups will have provided much of the
hardware understanding needed, and the software control
and optimization ends up becoming the critical final stage.
Although there are a number of programming options from
which to choose, the following are some of the most common
for which Keithley provides solutions.
Leverage SCPI commands to control the system
Using Standard Commands for Programmable Instruments
(SCPI) is most common for the majority of system integrators
who also write their own control code. SCPI commands are
provided in an easily readable ASCII format and help the
user logically map a command to the function or attribute
they wish to modify on the instrument in question. A series
of commands can be over the communications interface of
choice (USB, LAN, GPIB, or RS-232) to facilitate the hands-
free automation that is desired for production.
Although SCPI is sometimes referred to as a programming
“language,” it should be noted that the language in question is
one that is understood by test instrumentation, and should not
be confused with conventional programming languages (C++,
Visual Basic, and Python, for example). SCPI still requires
the use of a separate programming tool to allow for sending
commands and querying information.
IVI and LabVIEW instrument drivers
Instrument drivers provide a set of software sequences
that control a remote, programmable instrument used for
configuration, writing/reading, and triggering. These drivers
help to cut test program development time by minimizing the
user’s requirement for learning the programming conventions
for each instrument. IVI (Interchangeable Virtual Instrument)
drivers provide a generic wrapper around an instrument’s
specific SCPI command set, allowing for instruments from
different manufacturers to be used with the same driver.
For example, test engineers accustomed to using DMMs
produced by other companies could purchase a Keithley
DMM and integrate it into their existing test systems without
modifying their IVI-based control code.
Figure 24. Use KickStart for easy instrument configuration, applications focused testing, graphing, and data visualization.
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Because of the highly generic nature of pure IVI drivers and
the fact that instrument models provide custom features,
instrument designers will often provide a more specific IVI
driver that exposes the additional programming control
available for the selected instrument model. Although this
is in opposition to the promised interchangeability that
strict IVI intends to provide, these additional features tend
to offer the end user more than a one-size-fits-all model.
Keithley provides IVI-C and IVI-COM drivers for its DMM
and switching products that can be used with common
programming development tools such as Microsoft Visual
Studio, MATLAB®, Python and National Instruments’
LabWindows™/CVI. Installing the Keithley IVI drivers also
provides examples in C, Visual Basic, and C# to offer users
code that shows the actual application use cases for the
instrument driver.
LabVIEW® is a software-based graphical programming
environment that allows users to create programs showing
the flow of both variables and data (through wires), and
immediately have them presented on a user interface.
LabVIEW is a visual tool intended for control and data
presentation. It offers a building-block style that may
be easier for those who do not consider themselves
programmers to comprehend and become acclimated to than
traditional text-based programming languages like C and
Java. A program that would take several weeks to write using
a text-based programming language could be completed in
a fraction of that time using LabVIEW because it is designed
for on-screen measurement configuration, data analysis,
and results presentation. Because LabVIEW drivers use
VISA as the underlying instrument communication tool to call
underlying SCPI commands, the user also has the flexibility to
choose equipment with a variety of protocols, including GPIB
(General Purpose Instrument Bus), serial, USB, and Ethernet.
LabVIEW offers an extensive library function set to help users
with most programming efforts. Keithley provides LabVIEW
driver code libraries with application-specific examples
for the target instrument, which can include multi-channel
data acquisition, datalogging, single- and multi-point
measurement, digitized measurements, and switching.
Although the use of IVI and LabVIEW drivers promises easy-
to-use and ready-made tools, using either of these choices
still involves a learning curve. As noted, each leverages
an instrument’s SCPI command set, and optimizing a test
system almost requires users to become acquainted with
the lower-level commands. Additionally, some studies
indicate that direct use of an instrument’s SCPI command
set provides a speed advantage over the IVI and LabVIEW
options. Therefore, users need to weigh the advantages and
disadvantages of each before beginning their programming
efforts to ensure a balance of usability optimization for
themselves and other team members involved in the
development and maintenance of test system code.
Optimizing the system
For production test systems, it is important to investigate all
options that can decrease test time and increase throughput.
Presumably, at this point, the test equipment has been
chosen using the considerations outlined in “Guidelines for
Selecting a Data Acquisition System.” With the hardware
options covered and a clear understanding of the remote
commands needed to control the instruments, the next
task is constructing the software process that automates
product testing. Although an obvious solution to increasing
throughput might be to increase test system capacity, this
may be neither immediately feasible nor necessary. The
integrated programming environment (IDE) chosen probably
provides options to multithread or multitask; also consider
what the test instruments themselves offer with respect to
promoting test speed.
It is also important to weigh how much built-in instrument
intelligence the data acquisition system provides. Most
test instruments allow saving a number of configurations to
internal memory that can then later be recalled as needed.
This can greatly reduce the number of overall command
writes to the test instrument that the controlling PC would
have to execute; this would allow storing a long series of
commands on an instrument but only needing to send one
command to have them all executed.
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Data Acquisition Primer: An Introduction to Multi-Channel Measurement Systems
Now consider a series of instruments that must be set up
or that must execute their desired actions all at the same
time without the overhead of parallel processing on the
PC side. One way to implement this parallel operation is
through the triggering mechanisms that are available on the
test instrumentation used. A variety of options are possible,
either through dedicated trigger in and out ports, digital
inputs/output lines, or other communication protocol triggers
(such as LAN, GPIB, or PCI Express if these options are
available). Test instruments will usually have a documented
“trigger model” – a flow diagram that shows all the events
and actions that are staged to perform the switching and/
or measurement. Studying such models can help users
gain a deeper knowledge of the instrument-level processing
and may lead to insights on how to optimize the testing
scheme further. Even more advantageous is when the user
is presented with the ability to circumvent using a dedicated
trigger model and define their own to further remove any
unnecessary processing that might otherwise slow the
instrument.
Figure 25. Example of an instrument trigger model.
A variety of other options should be considered when looking
for ways to streamline testing:
• Use fixed ranging in lieu of auto ranging to remove any deciding logic that is applied in the instrument firmware. At this point, the anticipated test outcomes are already known so select a specific range and reap the benefits.
• Consider measurement quality vs. measurement speed tradeoffs. In a production setting where quantity may be valued over quality, it might be beneficial to reduce the integration time (or increase the sampling rate) to get results close to what the functional test is expecting. For example, reducing the aperture time from one power line cycle (1 PLC) to 0.01 PLC might not deliver the best 6½-digit resolution with an integrated measurement on a DMM, but it will deliver up to ten times faster readings.
• Leverage more mathematical and process control at the instrument level. If the test results require statistical information or data converted to values of a different unit of measure, use whatever built-in functionality the test equipment provides to help eliminate large data transfers and processor-taxing operations that might occur at the PC.
Many of these options will be standard features of the
instrumentation, with others available as add-ons. Others
might only be available through specific vendors, so take the
time to seek out instrumentation that offers the capabilities
most important to a particular application.
For example, Keithley instruments with Test Script Processor
(TSP®) technology operate like conventional instruments
by responding to a sequence of commands sent by the
controller. Individual commands can be sent to a TSP-
enabled instrument the same way as any other instrument.
TSP technology allows message-based programming, much
like SCPI, with enhanced capabilities for controlling test
sequencing/flow, decision-making, and instrument autonomy.
Unlike conventional instruments, TSP-enabled instruments
can execute automated test sequences independently,
without an external controller. A series of TSP commands can
be loaded into the instrument using a remote computer or via
the front-panel port with a USB flash drive. These commands
can be stored as a script that can be run later by sending
a single command message to the instrument, or loaded
and run from the front panel. The primary tradeoffs are
programming complexity vs. throughput. Relatively simple
programming techniques will allow noticeable throughput
improvements. Slightly more complex programming (using
TSP-based instruments and programming best practices) can
produce significantly higher throughput gains.
See Appendix B for examples of software options available
for various Keithley data acquisition tools.
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Example Application
Performing a long-term temperature scan is a very
common application for data acquisition instruments. This
example employs a DAQ6510 with a 7700 Plug-In Switch
Module for the data acquisition system, which will monitor
the temperature on specific locations of a product in a
temperature chamber.
Figure 26. Temperature profiling in your lab.
• To keep the cost of the solution low while still covering a broad range of temperatures across five channels, the transducer of choice is the thermocouple.
• A Type K thermocouple type is used because it supports the widest range of temperatures; internal cold junction correction is employed to enhance the measurement accuracy by compensating for offsets that may present themselves at the connection of the thermocouple to the measurement channel.
• Thermal profiling of a device under test (DUT) can last for days, weeks, or up to a year. This example will run for 24 hours.
• Because lab setups can have limited resources, the setup will assume no PC is available, but the measured data must still be preserved. Therefore, the data will be exported to a USB flash drive for each of the scans executed.
Here is a general diagram of the test setup:
Switch Card
Meter
DAQ6510EnvironmentalChamber
DUT
Figure 27. Block diagram of a potential temperature profiling setup.
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Data Acquisition Primer: An Introduction to Multi-Channel Measurement Systems
1. Power cycle the instrument and ensure the TERMINAL switch is set to REAR. Build a scan and Add Groups of Channels, inclusive of 101 to 105 for Temperature.
2. Scroll down in the Settings tab and
ensure that the Transducer is set to Thermocouple and the Type is set to K.
3. Touch the Scan tab and set the Scan
Count to 1440, and set the Interval Between Scans to 60s. NOTE: Why a count of 1440? 24 hrs * 60 sec = 1440
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2
3
1
2
3
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4. Scroll down in the Scan tab to locate the Export to USB option. Touch the adjacent button, set to After Each Scan, and accept all the default settings presented in the resulting dialog by touching OK.
5. Insert a USB flash drive into the port
on the front of the instrument.
4
5
4
5
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Data Acquisition Primer: An Introduction to Multi-Channel Measurement Systems
6. Touch the Start button to begin the scanning process.
7. Either during or upon completion of the scan, the operator may desire to view the acquired data graphically. To do so, navigate to MENU->Views->Graph Use the menu control to change the channels being viewed.
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6
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ConclusionUnderstanding the intricacies of data acquisition can be quite
a complex task. It is essential to determine the scale of the
system required in terms of channel count and the accuracy
of measurement hardware, as well as the switching type
and topology that will be used to route the signals. The next
step is understanding the types of measurements required
for the application and seeking guidance on how and when
to apply optimization techniques to those measurements.
It is also important to consider all the software options for
the equipment in question for applications that must make
a transition from the lab to the production floor. Study
everything that is available in the way of documentation—the
application examples, demos, and videos, etc.—not just the
standard manuals. Finally, consider the resources available
to offer assistance with questions that the documentation
cannot answer.
References1. A. Clary, “IVI drivers: Slower but simpler,” EDN, February
1, 2003. [Online], Available: https://www.edn.com/design/test-and-measurement/4383376/IVI-drivers-Slower-but-simpler. [Accessed: Nov. 20, 2017].
2. Data Acquisition and Control Handbook: A Guide
to Hardware and Software for Computer-Based
Measurement and Control, 1st edition, Keithley
Instruments, 2001.
3. Switching Handbook: A Guide to Signal Switching
in Automated Test Systems, 6th edition, Keithley
Instruments, 2008.
4. Low Level Measurements Handbook: Precision DC
Current, Voltage, and Resistance Measurements, 7th
edition, Keithley: A Tektronix Company
Appendix A: Keithley Data Acquisition Systems Overview2750 3706A DMM6500 DAQ6510
Number of Slots 5 6 1 2
Total channels 200 576 10 80
Communication GPIB, RS-232 USB, LAN, GPIB USB, LAN USB, LAN
User Interface Menu buttons Menu buttons Touch Screen Touch Screen
Internal buffer 110,000 readings 650,000 readings 7,000,000 readings 7,000,000 readings
Software KickStart LabVIEW driver KickStart KickStart
Triggers Trigger Link Digital I/O BNC Ext In, Trig Out BNC, Ext In, Trig Out
Digital I/O Yes Yes Yes Yes
Command set SCPI TSP SCPI, TSP SCPI 2000, SCPI 34401
SCPI, TSP, SCPI 2700, SCPI 2701
Scan rate 210 chan/sec 1250 chan/sec >90 chan/sec >1000 chan/sec
Measurement rate 2000/sec 14,000/sec >15,000/sec >15,000/sec
Appendix B: Software Support Tools Compatible with Keithley InstrumentsTouchscreen
InterfaceEmbedded
Web InterfaceSCPI
Commands TSP Enabled KickStart IVI DriverLabVIEW
Driver
27xx ■ ■ ■ ■
Series 3700A ■ ■ ■ ■ ■ ■
DMM7510 ■ ■ ■ ■ ■ ■ ■
DMM6500 ■ ■ ■ ■ ■ ■ ■
DAQ6510 ■ ■ ■ ■ ■ ■ ■
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Data Acquisition Primer: An Introduction to Multi-Channel Measurement Systems
Primer
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